Method of fabricating large vessels capable of withstanding high internal pressures

ABSTRACT

HOLLOW THICK WALLED CYLINDERS SUITABLE FOR WELDING TOGETHER TO MAKE LARGE VESSELS THAT WILL WITHSTAND 1000 TO 5000 POUNDS PER SQUARE INCH INTERNAL PRESSURES ARE MADE BY CASTING A HOLLOW CYLINDER, OUT OF STEEL, THAT HAS TWICE THE WALL THICKNESS THAT IS DESIRED. THEN THE CASING IS CUT LONGITUDINALLY INTO TWO HOLLOW CYLINDERS OF THE DESIRED ONE-HALF WALL THICKNESS OF THE CAST CYLINDER BY ELECTROCHEMICAL MACHINING. THIS BRINGS ANY INTERNAL CAVITIES TO THE SURFACE ON THE TWO CUT APART CYLINDRICAL SHELLS AND THE CAVITIES CAN BE CHEAPLY REPAIRED BY GRINDING AND WELDING.

United States Patent 3,557,437 METHOD OF FABRICATING LARGE VESSELSCAPABLE OF WITHSTANDING HIGH IN- TERNAL PRESSURES John C. St. Clair, Box333, RR. 2, London, Ohio 43140 No Drawing. Filed Jan. 14, 1969, Ser. No.798,854 Int. Cl. B23k 19/00; B23p 17/00 US. Cl. 29-415 6 Claims ABSTRACTOF THE DISCLOSURE The purpose of this invention is to provide cheapvessels capable of withstanding internally the high pressures that areneeded in processes for converting coal into natural gas, fuel oil,gasoline and other products. In practically all processes for producingthe preceding valuable products from coal it is necessary to react a gasunder high pressure [with coal as the first step of the process. Thedevelopment of cheap high pressure 'vessels would not only cheapen thecost of this expensive initial high pressure step but it would alsoallow lower temperatures to be used, which will produce normally higheryields of desired products, since then the longer reaction times andlarger high pressure vessels necessary would not be too expensive asthey are now. Of course high pressure vessels are used by many otherindustries and my invention supplies these.

In the usual high pressure process it is economical to design the highpressure vessel used as relatively long as compared with its diameter.This makes the construction of the long cylindrical shell the major itemof cost. (I will later show a cheap method for making the ends that evenreduces the ends present low relative cost.)

A long cylindrical shell is normally made of several shorter shells orrings welded together by girth welds. The making of these shorter shellshas in the past been very expensive. To withstand high internalpressures the thickness of the metal walls becomes too thick to allowthe shell to be made out of metal plates bent into cylindrical form, forwelding, by cheap conventional methods. Therefore expensive multiplewalled constructions have been normally used at internal vesselpressures of over 3000 pounds per square inch or greater.

Casting the vessels out of molten metal, poured into molds, on thesurface has looked attractive but there have been encountered severedifficulties in casting high pressure vessels in the past. The mainobjection is the expense of obtaining sound castings, lwithout internalcavities, at a reasonable cost. For the usual vessel of large size, thevessel has a large external area. This requires a large number of risersto be used to supply more molten metal to the casting while it issolidifying. This is necessary since normally molten metal andparticularly molten steel decrease in volume when they solidify and itis necessary to keep supplying molten metal to the casting while itsolidifies. Since the molten metal cannot be depended on to flow farwhen the casting is undergoing the critical stage when it has almostcompletely solidified there must be a large number of risers undernormal casting conditions. Another difiiculty, that arises with thethick walls of a cast high pressure vessel, is what to do if therehappens to be a cavity in the final cast walls. These cavities can bereadily located by known methods but the objectionable cavity willnormally be in the center of the thick wall. The standard procedure ofgrinding down to the cavity and then welding the hole full of metalbecomes very expensive. Also weld metal is never as strong as theoriginal metal and this produces an undesired last minute reduction ofthe internal pressure that the vessel will stand. Therefore to preventhaving to throw any vessel with cavities away it is necessary tooverdesign all vessels to start with. Now with smaller outside areas andthe resulting smaller number of risers and with only part of the metalto undergo high stress as is encountered in average castings theprobability that at least one of the risers will not work well enoughcan be reduced to a small figure by conservative riser design. Howeverwhen one has so many risers for one casting and when it is desirable tohave all the metal stand the maximum possible stress as it is with highpressure withstanding vessels the expense of obtaining all solid metalby normal methods becomes high.

In my disclosed invention I solve the previous problems of casting thickwalled high pressure vessels by initially casting a very thick walledcylindrical shell or shape and then longitudinally cutting the verythick walled cylindri cal shape into at least two concentric cylindricalshapes. As for example I might cast a cylindrical shell 10 feet indiameter on the outside and 9 feet in diameter on the inside. The wallof the large cylindrical shape would be constant throughout and would be6 inches thick. Then I would cut the large cast cylindrical shape intotwo concentric desired shells. The larger shell would be 10 feetdiameter on the outside and 9 feet-6.25 inches in diameter on theinside. The smaller cylindrical shell produced would be 9 feet-5 .75inches in diameter on the outside and 9 feet in diameter on the inside.Both cylinders would each be equal in length to the originally castlarger cylinder.

The method used for cutting the larger hollow cylinder originally'castinto two or more smaller cylinders is conventional. Probably the bestmethod would be to use electrochemical machining. This method isdescribed in Standard Handbook for Mechanical Engineers by Baumeister &Marks, 7th ed., McGraw-Hill, New York, pp. 13-103 and 104. Very brieflythis method consists of a reverse electroplating process. That is thedirection of the flow of direct current is reversed, from its directionof travel in electroplating, so that the metal is eaten away from ametal object instead of plating on the metal object.

In electrochemical machining water is supplied to the area where themetal is eaten away both to carry a desired electrolyte to carry theelectrical current but more particularly to carry away the heat of thereaction. The rate of eating away of the metal is carried out at a fastrate so that the distance between the electrode (carrying the current tonear the metal that is eaten away) to the metal is the controllingfactor rather than the reaction on the surface that eats away the metal.Therefore by keeping the electrode very close to the metal, the velocitythat the metal is eaten away at any relatively long distance from theelectrode is small and with insulation the over cut of the process tothe sides is normally of the order of 0.015 inch though this can easilybe made larger if desired. In my immediate case the electrode and theduct carrying the water solution of the electrolyte would be in the formof a very thin walled hollow cylinder. The electrode would be coveredwith plastic paint to limit the eating away of the metal to at only theend of the electrode. The cutting device would look like an enormousbiscuit cutter.

Another method that is cheap is to make the casting out of easilymachined metal and then with sharp tools on the bottom of walls of avery thin walled hollow metal cylinder cut or machine the large castcylinder into several thinner walled concentric cylinders. In this casethe cutting device would look like an enormous biscuit cutter that isprovided with means for rotating while it cut the biscuits.

Instead of sharp tools on the bottom of the very thin walled hollowcylinder used for cutting, pieces of bonded abrasives could be used.Then the metal removed in cutting the meal cylinder would be removed bygrinding. This latter grinding procedure would be preferably preformedby my copending patent application Ser. No. 761,539 in which theabrasive would be bonded by cast copper and the abrasive material wouldbe periodically sharpened by removing dulled grit by contacting thecopper bonded abrasive with an aerated water solution of ammonia whichdissolves copper without corroding iron. In this case the cutting devicewould look like an enormous biscuit cutter with means to rotate thebiscuit cutter while cutting.

The preceding process of casting a thicker walled hollow cylinder firstand then cutting it up into a plurality of concentric thinner walledhollow cylinders has the advantage that it only takes the constructionof one mold and one core to make a plurality of final hollow cylinders.But of more importance is that the number of risers needed for thesingle mold is only a fraction of the number needed for each moldrequired if the thinner walled cylindrical shells were cast separately.The number of risers needed for a casting is inversely proportional tothe wall thickness cast. That is if a conventional casting process for asingle thinner walled hollow cylinder required 8 risers, then to castthe same diameter and height but with a wall twice as thick there wouldonly be needed 4 risers. And besides with a wall twice as thick therewould be, after cutting them apart, two thinner walled cylinders made.In other words in this example the required number of risers per finalshell produced would be reduced 75% by my method.

However, the most important advantage of my invention is that when thethicker walled cast cylinder is cut up into a plurality of thinnerwalled hollow cylinders any cavities in the walls of the originalcasting become very easy to repair. This is because, in a casting of thepreceding type, cavities about always occur in the center of the wall ofthe casting. Therefore when the casting is cut in just two equally thickthinner walled cylinders the cavities are on the surfaces of thecut-apart cylinders. Therefore the cavities can be easily welded full ofmetal. And there will not be the usual large amount of grinding out ofmetal for holes with its resulting filling in of large amounts of weldmetal that somewhat weakens the casting.

In case the thinner walled cylinders are not cut apart near the centerline of the walls of the thick walled cast cylindrical shell in allcases the cutting procedure will place the cavities nearer to a surfaceand facilitate repair of the cavities.

As I have said before, the big expense normally for making thick walledhigh pressure vessels is the cost of making the cylindrical shell.However, due to recent developments even the cost of the ends have beenmade cheaper. In this case an end of the vessel is cast with its wallover twice as thick as is required. Then the outer surface is grounddown to the proper thickness by my method of grinding with copper bondedabrasive as I have described previously in this application. Theabrasive will be in the form of a grinding wheel of course instead of agiant biscuit cutter. In this grinding method all cavities will beremoved from the cast end of the vessel and the metal ground off can berecovered unharmed for remelting and reuse.

In conclusion I may say that I have disclosed a method for castingcylindrical shells or shapes by casting a thicker walled cylindricalshell or shape and then cutting the thicker walled cylindrical shapeinto a plurality of thinner walled cylindrical shapes. My method greatlydecreases the cost of the molds and risers required. It also greatlycheapens the cost of repairing cavities and greatly reduces the amountof overdesign necessary to prevent rejection of castings on the accountof cavities.

The longitudinal axis of a cylinder is the straight line joining thecenters of the two ends of the cylinder.

In addition it may be said that with a relatively small investment thata manufacturer of steel castings may go into the business of castingrings and heads for thickwalled vessels that normally sell for as muchas $100,000 apiece. All the company has to do is to purchase theelectrical equipment and the etching liquid handling equipment for a3000 ampere electrochemical machining apparatus which costs less than$30,000. The so called biscuit cutter shaped electrode or tool assemblycan be home made and made by spacing copper tubes placed longwise, andspaced for example 3 inches apart, on an expendable drum form. Betweenthe copper tubes are placed copper strips laying lengthwise. Then thesides of the copper tubes are brazed to the edges of the copper stripsand with brazing on a large flat end, to the formed cylinder, thus theso called biscuit cutter shape is made. The drum around which thebiscuit cutter shape is made is removed and the biscuit cutter shape iscovered with an electrical insulating paint over all but the ends of thecopper tubes that do the actual electrochemical machining. The biscuitcutter shape or tool is slowly rotated around its longitudinal axiswhile the tool cuts a large cast hollow cylinder into two hollowcylinders. The ends of the copper tubes that do the actualelectrochemical machining can have plastic guards put in front of themin the path of rotation so that the copper tube ends wont actually touchthe metal cut and cause short circuits which are harmful.

Heads for the thick walled vessels can be made two at a time by cuttinga head cast with twice the desired thickness of metal. The casttwice-as-desired thickness head is cut into two heads using the abovebiscuit cutter shaped tool that is modified. In this case the biscuitcutters cutting edge is cut itself into longitudinal strips that aregiven an angle inward towards the biscuit cutters longitudinal axis. Thelongitudinal strips are made with a clearance between them and are mademovable in relation to the flat end of the biscuit cutter but with theangle of the strips with the flat end of the biscuit cutter being heldconstant. Then by moving the longitudinal strips of the cutting edge ofthe biscuit cutter slowly inward at the angle above mentioned, and whichis by moving the longitudinal strips relative to the fiat end of thebiscuit cutter, a cone shaped surface can be cut. If the longitudinalstrips are bent in the shape of an arc of a circle, a vessel head in theform of a hemisphere can be cut.

I claim:

1. The method for casting hollow cylindrical metal shells for theconstruction of vessels that withstand internal pressures whichcomprises: casting a large hollow cylindrical metal shell whose wallthickness is sufficiently thick from which to make a plurality ofdesired cylindrical shells, cutting said large metal shell into aplurality of metal shells each having wall thicknesses in excess of oneinch by cutting said large shell at a constant distance from itslongitudinal axis, repairing the cavities which exist in the cut-outmetal shells, and using each of the plurality of cut-out metal shells inthe construction of a vessel that is capable of withstanding internalpressures in excess of 200 pounds per square inch gage pressure.

2. A method according to claim 1 in which the metal is an alloy of iron.

3. A method according to claim 2 in which the cutting is done byelectrochemical machining.

6 4. A methodaccording to claim 2 in which the cutting References Citedis done by grmdmg- UNITED STATES PATENTS 5. A method according to claim3 in which said large metal shell is cut into two cylindrical shells insuch a manner that neither cut-out shell has a wall thickness that ismore than 20% greater than the wall thickness of the z d d l h JOHN F.CAMPBELL, Primary Examiner met 0 accor ing to c aim 4 in w ich saidlarge metal shell is cut into two cylindrical shells in such a man-REILEY Asslstant Exammer ner that neither cut-out shell has a wallthickness that 10 5 CL th 20 t h l ghlrggrscilenan grea er t an the walthickness of the 29 402, 527.6; 164 69 1,606,282 11/ 1926 Witter.2,184,183 12/1939 Fykse 29-416 5 3,142,115 7/1964 Schaming 29416

